Position an oven rack in the center and preheat the oven to 325°F (163°C).

Add 16 Oreo cookies to a food processor and pulse until finely crushed.

Add the melted butter and granulated sugar, then pulse until the crumbs are evenly moistened.

Transfer the cookie mixture to a 9-inch pie plate. Press it firmly and evenly across the bottom and up the sides.

Bake the crust for approximately 15 minutes, or until fragrant. Transfer to a wire rack to cool.

To prepare the brownie layer, combine the bittersweet chocolate, butter, vegetable oil, and cocoa powder in a microwave-safe bowl.

Microwave at 50% power in short intervals, stirring frequently, until completely melted and smooth.

In a separate mixing bowl, whisk together the dark brown sugar, eggs, alcohol-free vanilla flavoring, and salt until smooth.

Gradually whisk in the melted chocolate mixture. Add the flour and gently stir just until incorporated.

Pour the brownie batter into the prepared cookie crust and spread it evenly.

Bake for approximately 15 minutes, or until the edges are set but the center remains fudgy. A toothpick inserted near the center should have a thin coating of moist batter rather than coming out completely clean.

Cool on a wire rack for approximately 1 hour. Transfer the pie to the refrigerator and chill for at least another hour, or until completely cold.

Meanwhile, prepare the cookie topping. Line a rimmed baking sheet with parchment paper.

Crush the remaining 6 Oreo cookies into roughly 1/2- to 3/4-inch pieces.

Combine the cookie pieces with powdered sugar, cocoa powder, and salt. Stir in the melted butter until moist clumps form.

Spread the mixture over the prepared baking sheet. Bake at 325°F for approximately 10 minutes, stirring halfway through. Cool completely and break apart any oversized clusters.

Once the brownie layer is completely chilled, prepare the mousse. Place the finely chopped milk chocolate in a microwave-safe bowl and microwave at 50% power in short intervals, stirring frequently, until smooth.

Allow the melted chocolate to cool until warm rather than hot, ideally around 90–100°F (32–38°C).

Heat 3 tablespoons of the heavy cream until warm, approximately 105–110°F (40–43°C). Whisk in the cocoa powder until completely smooth.

Add this cocoa mixture, powdered sugar, salt, and the remaining cold heavy cream to a mixing bowl.

Beat on medium speed until the cream begins to thicken. Increase to high speed and continue beating for 2–3 minutes, or until soft peaks form.

Fold approximately one-third of the whipped cream mixture into the melted chocolate to lighten it.

Gently fold in the remaining whipped cream until evenly incorporated, taking care not to deflate the mousse.

Spread the chocolate mousse evenly over the completely cooled Mississippi Mud Pie.

Sprinkle the cooled chocolate cookie crumble over the mousse.

Refrigerate for at least 3–4 hours before slicing. For the best texture and cleanest slices, chill overnight